Definition of "brier"

Adeline thought little about it at the time; but often—oh! how often—did her memory dwell upon those accents, and recal that beloved voice, when after-events came thickening like briers in her path of life.

1842, [Katherine] Thomson, chapter V, in Widows and Widowers. A Romance of Real Life., volume II, London: Richard Bentley, […], page 118